Photo-oxidation and photo-reduction in alkylthiol monolayers self-assembled on gold

Photo-oxidation and photo-reduction have been observed in self-assembled monolayers (SAMs) of 8-chlorooctyldisulfide adsorbed on gold. Using the techniques of X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) we were able to distinguish oxidation at the gold/sulfur interface and reduction at the chlorine/air interface, in an SAM of 8-chlorooctyldisulfide pre-exposed to an ambient environment of light and air; but no such evidence was seen in an identical sample kept in the dark with the exclusion of air. A second study, in which the near-edge X-ray absorption fine structure (NEXAFS) of octane-1,8-dithiol self-assembled on gold foil was compared with that of 8-bromooctyldisulfide and octane-l-thiol, gave compelling evidence that photo-oxidation is mediated by the gold surface. Since much of the interest in self-assembled monolayers centres on the many potential applications of such ordered layers of tailored functionality, it is important to note that their usage may be restricted to applications in which the monolayer is kept either in the dark or under anaerobic conditions.